1. Field of the Invention
The present invention relates generally to a voltage generating circuit, and particularly to a voltage generating circuit that generates an output voltage according to an input voltage.
2. Description of the Related Art
An audio amplifier circuit that amplifies an audio signal and outputs the amplified signal to a speaker or a headphone is known in the conventional art.
Such an audio amplifier circuit has a shutdown function and a mute function for cutting noise upon turning the power on/off.
FIG. 1 is a circuit diagram showing an exemplary configuration of a conventional audio amplifier circuit 101.
In this audio amplifier circuit 101, an input signal is supplied to an input terminal Tin via a condenser C41 that cuts direct current from a signal source 102. The input signal supplied to the input terminal Tin is then supplied to an amplifier circuit 111. The amplifier circuit 111 includes a differential amplifier circuit 121, an input resistor R31, a return resistor R32, and a switch 122. The amplifier circuit 111 corresponds to an inverting amplifier circuit that receives a standard voltage from a standard voltage generating circuit 112.
The amplifier circuit 111 outputs a signal according to a difference between the standard voltage from the standard voltage generating circuit 112 and the input signal supplied to the input terminal Tin. The signal amplified at the amplifier circuit 111 is output from an output terminal Tout to drive a speaker 103.
The switch 122 is implemented between a connection point of the input resistor R31 and the return resistor R32, and an inverting input terminal (−) of the differential amplifier circuit 121, and this switch 122 turns on/off by a mute signal that is supplied to a control terminal Tcnt1 from a controller 104. In this way, the supplying of the input signal to the differential amplifier circuit 121 may be controlled, and in turn, the mute function may be controlled.
The standard voltage generating circuit 112 includes a switch 131, resistors R41 and R42, and a condenser C51. A fixed voltage Vdd is applied to the standard voltage generating circuit 112. Specifically, the fixed voltage Vdd is applied via the switch 131 to a series circuit that includes the resistors R41 and R42. The switch 131 turns on when a shutdown signal supplied from the controller 104 to a control terminal Tcnt2 is at a high level, in which case the fixed voltage Vdd is applied to the series circuit including the resistors R41 and R42. The switch 131 turns off when the shutdown signal is at a low level in which case the supplying of the fixed voltage Vdd to the series circuit including the resistors R41 and R42 is stopped.
When the switch 131 turns on, the resistors R41 and R42 divide the voltage Vdd, generate the standard voltage, and supply the generated standard voltage to a non-inverting input terminal (+) of the differential amplifier circuit 121. In this way, the amplifier circuit may be in an operating state. It is noted that, a terminal Tc is connected to a connection point of the resistor R41 and the resistor R42, and the terminal Tc is connected to a condenser C51 that is implemented outside the audio amplifier circuit 101. The condenser C51 externally connected to the terminal Tc absorbs ripples of the standard voltage.
FIGS. 2A˜2E are diagrams illustrating the operation of the audio amplifier circuit 101. FIG. 2A represents the shutdown signal that is output from the controller 104, FIG. 2B represents the switching state of the switch 131, FIG. 2C represents the standard voltage that is supplied to the differential amplifier circuit 121, FIG. 2D represents the mute signal that is output from the controller, and FIG. 2E represents the switching state of the switch 122.
When the level of the shutdown signal changes from low to high at time t10 as is shown in FIG. 2A, the switch 131 turns on as is shown in FIG. 2B. When the switch 131 turns on, the standard voltage is generated by the resistors R41 and R42. In this case, the standard voltage gradually rises according to the charge voltage of the condenser C51, and reaches a predetermined level at time t11 as is shown in FIG. 2C. When the standard voltage reaches the predetermined level at time t11, the differential amplifier circuit 121 may be ready for operation.
The controller 104 counts up the time from when the shutdown signal is switched to a high level, and, after a predetermined amount of time elapses, outputs a mute signal at time t12, as is shown in FIG. 2D. With the output of the mute signal, the switch 122 of the amplifier circuit 111 turns on, as is shown in FIG. 2E, and the mute state of the input signal is disengaged so that the input signal may be amplified at the amplifier circuit 111 and supplied to the speaker 103.
As can be appreciated from the above description, in the audio amplifier circuit 101, the generation of the standard voltage at the standard voltage generating circuit 112, the operation of the amplifier circuit 111, and the shutdown function of the amplifier circuit 111 are controlled based on the shutdown signal from the controller 104. Further, the mute function of the amplifier circuit 111 is controlled based on the mute signal from the controller 104.
In another example, an audio amplifier circuit that controls a generation of a standard voltage of an amplifier circuit according to a standby signal is proposed in U.S. Pat. No. 5,642,074.
However, in the conventional audio amplifier circuit, there is a delay in the rise of the standard voltage with respect to the input of the shutdown signal owing to the condenser C51 for absorbing ripples.
Thus, the rise time of the standard voltage is lengthened, and the audio output is delayed.